This invention relates to an electronic endoscope system having an inter-line-reading type of solid image pickup device which effects photoelectric conversion of a subject image to obtain an image signal.
Recently, electronic endoscopes incorporating at their tips an image sensor such as a CCD from which an endoscope image is output as a TV signal are replacing conventional fiberscopes having an image guide formed of a bundle of optical fibers and a hand operation unit including an ocular portion through which a subject is observed.
For electronic endoscopes, an image sensor having a very small sensing area is required, because it is disposed in an observation head portion in place of the image guide. An R/G/B-surface-sequential image pickup method is known as a method of optimizing color resolution of the total number of image sensor picture elements arranged in the restricted area.
Examples of conventional endoscopes will be described below with reference to FIG. 18 to 24.
As shown in FIG. 18, an electronic endoscope 150 of this kind has an internal filter plate 162 which has color filters having colors R, G, and B and which is disposed on an emergence optical path of a white light source 160. The internal filter plate 162 is rotated to successively irradiate a subject (not shown) with light of these colors through a light guide 156 in the electronic endoscope 150, and image pickup signals obtained from an image sensor 158 with respect to the colors are converted by an A/D converter to extract image signals which are sent to D/A converters 172, 174, and 176 through image memories, R memory 166, G memory 168 and B memory 170, to be combined, thereby obtaining a color image on a color CRT screen (not shown).
For this process, on the TV image output (CRT) side, Fields A and B are interlaced for prevention of flicker. The applicant of the present application has proposed a surface-sequential image output method for a process including this interlacing in Japanese Patent Laid-Open Publication No. 62-82888.
That is, a CCD image sensor of a frame transfer type or a line reading type is used, and image signals 1, 2, 3, . . . n are thereby generated with respect to scanning lines are successively stored in frame memories. After groups of information on frame images in colors R, G and B have been stored in the memories, each of them is read every other line with respect to the frames in an alternative field reading manner (signals 1, 3, 5 ... are read in field A and signals 2, 4, 6 . . . are read in field B) for interlace scanning. It is thereby possible to obtain an image while reducing the magnitude of flicker.
In this case, however, since one color image is formed from three frame images R, G, and B, flickering and color misalignment take place inevitably due to time differences therebetween.
An apparatus designed to avoid this drawback has been developed which uses an image sensor having a color mosaic filter whose elements are respectively put on pixels on the image sensor sensitive surface to obtain three color signals at a time. Needless to say, in this case, the color resolution and sensitivity are sacrificed in comparison with the former.
A CCD having a mosaic filter (single plate color CCD) will be described below. In the case of a single plate color type, the aperture rate of the sensitive portion is small in comparison with the frame transfer type and the line reading type (each adapted for the black-and-white image pickup and surface-sequential color methods alone), and the exposure time necessary for obtaining one color image is short and the sensitivity is therefore low, as mentioned above. To mitigate these drawbacks, a complementary color filter having cyan, magenta, yellow elements having optical transmissivities higher than those of the R,G, B primary color filters is put on the CCD.
A color separation process used in such a case will be described below with reference to FIG. 19.
For field A reading at line 180, a signal from alternate pixel portions Mg, Cy, Mg, Cy and a signal from alternate pixel portions Yel, G, Yel, G are output simultaneously and added. Accordingly, the output at line 180 is given as a cycle of 2R+B+G, B+2G, 2R+B+G, and B=2G and is expressed as an AC component (2R - G) sin.omega.t.
Similarly, at line 182, signal outputs from alternate pixel portions Mg, Cy, Mg, Cy and signal outputs from alternate pixel portions G, Yel, G, Yel are added to obtain (2B - G) sin.omega.t. From these AC components, color difference components 2R - G, 2B - G are extracted by carrier component demodulation. Signals for line 184 and subsequent lines are processed in the same manner.
With respect to field B, 2R - G and 2B - G are alternately obtained at lines 186 and 188. The combination of pixel rows for each line is shifted one row between the fields to adjust interlace scanning and spatial positions at the time of television image output to reduce the magnitude of flicker in the5 image output. Signals for line 190 and subsequent lines are processed in the same manner.
For charge transfer in complementary color filer CCDs, inter-line reading methods based on two-line simultaneous reading are ordinarily practiced. The concept of them will be described below briefly with reference to FIGS. 20 and 21. FIG. 20 shows the principle of the operation of a typical two-line simultaneous reading, and FIG. 21 shows the principle of the operation of vertical transfer based on typical four-phase driving.
In FIG. 20, blocks 200, 202, 204, 206, and 207 represent photodiodes, and blocks 208, 210, 212, 214, 216, 218, 220, 222, and 224 represent buckets for reading charges on the photodiodes. Charges on the photodiodes 202 and 206 adjacent to buckets 212 and 220 are first read into the buckets 212 and 220 by timing B, and charges on the photodiodes 200 and 204 adjacent to the buckets 208 and 216 are read into the buckets 208 and 216 by timing H and are mixed with the charges in the buckets 212 and 220. Charges on adjacent photodiodes (photodiodes 200 and 202, 204 and 206) are thus added. At the time of reading in the next field, different combinations of photodiodes having charges to be added (photodiodes 202 and 204, 206 and 207) are set. Interlacing is therefore effected at the time of charge reading from the CCD.
As shown in FIG. 21, charges read out by two-line simultaneous reading are transferred from left to right by changing the bias voltages for the buckets are successively changed. This is the principle of the operation of vertical transfer based on typical four-phase driving. Reference symbols in FIG. 21 are the same as those in FIG. 20.
A CCD having such a structure is incapable of one-line sequential reading because charges are mixed owing to its specific structure. Transfer for one-line sequential reading cannot be performed unless at least three registers are provided for one photodiode.
A three-phase driving method using at least three registers for one photodiode for such reading requires at least three registers for each photodiode, for example, as shown in FIG. 22, registers 226, 228, and 230 for photodiode 200, registers 232, 234, and 236 for photodiode 202, registers 238, 240, and 242 for photodiode 204, and registers 244, 246, and 248 for photodiode 207, so that the register capacity per unit, i.e., the maximum transfer charge amount is reduced, resulting in a reduction in the image sensor saturation limit.
For this reason, in the case of an image sensor, such as a two-line simultaneous reading image sensor of the above-mentioned inter-line type and of the non-interlace-adaptation type, having a structure incapable of one-line reading, if it (of course, a black-and-white image sensor having no color filter) is adapted for the R/G/B-surface-sequential method, one (one frame of) color image is formed by exposure for two fields for each color, i.e., for six fields in all.
That is, 2:1 interlace scanning is effected, color video signals are formed with respect to fields of odd scanning and even scanning, odd and even field signals for each color are combined to form one frame image in the corresponding color, and one frame of color image is formed from three frame images in R, G, and B. Among methods of forming a static color image, one shown in FIG. 23 is the simplest. There is another example of the color static image formation method shown in FIG. 4 in which a one frame of color image is formed by a combination such that exposure color is sustained in the opposite fields of one frame.
If these methods are directly applied to processing of an animated image, two fields, i.e., 1/10 second is required to obtain one frame of color image. It is not easy for a viewer in observing a movement of the animated image with a flicker at 10 Hz thereby generated. In the case of the R/G/B-surface-sequential method, along with the flicker or movement in a color frame cycle, a color misalignment phenomenon due to a deviation of the color exposure timings caused when the subject is moved occurs because the exposure timings for the respective colors are sequential (continuous) with respect to time. There is therefore the problem of fatigue of visual sensation during observation.